1. Field of the Invention
The present invention relates to a dip coating apparatus and more particularly, to a dip coating apparatus including a jig having a function of generating a stream of air directed downward toward the surface of a coating liquid stored in a bath and causing the jig to hold a base to be coated and dip it in the liquid for forming a film on the base.
2. Discussion of the Background
A dip coating method is extensively used to coat a hollow cylindrical base with a Iiquid for forming a photoconductive element. The problem with the dip coating method is that a film formed on the base sags and becomes thinner at the upper end portion of the member, at which coating begins, than the other portion. While various methods have heretofore been proposed for obviating sagging, they need sophisticated coating apparatuses, often render the resulting films irregular in thickness, and cannot fully obviate sagging.
Japanese Patent Laid-Open Publication No. 59-127049, for example, teaches a method which reduces the vapor of a solvent produced from a photoconductive coating liquid when a hollow cylindrical base is lifted out of the liquid. This method, however, needs an exclusive air passage mounted on the top of a bath for reducing the density of the above vapor and is not practicable without resorting to a complicated coating apparatus. Moreover, because air is caused to flow toward the base sideways, it hits against the base in a particular manner at each of the upstream side and downstream side and is apt to render the resulting film irregular.
Japanese Patent Laid-Open Publication 59-225771 discloses a method using an annular air doctor positioned above the surface of a coating liquid stored in a bath. A stream of air is sent from the air doctor to a hollow cylindrical base when the base is lifted out of the liquid. This also renders the resulting film irregular due to the disturbance to the stream of air inside the bath and the stream of air directly hitting against the film.
Japanese Patent Laid-Open Publication No. 63-7873 teaches an arrangement in which a flexible hood is positioned on the top of a bath for sequentially reducing the density of vapor downward. This kind of arrangement is not practical without resorting to a complicated apparatus.
Japanese Patent Laid-Open Publication No. 1-107874 teaches that air is caused to flow horizontally when a hollow cylindrical base is lifted. This also has the problem stated in relation to the above Laid-Open Publication No. 59-127049.
Japanese Patent Laid-Open Publication No. 3-213171 proposes a method that blows, when a hollow cylindrical base is lifted, air swirling in the circumferential direction above a bath. This method also needs a complicated apparatus and is apt to render the resulting film irregular because air hits against the base.
Further, Japanese Patent Laid-Open Publication No. 4-29773 discloses a method which blows air from a nozzle in a direction tangential to a hollow cylindrical base when the base is lifted. This air, however, causes a liquid to flow on a part of the base against which air hits, also rendering the resulting film irregular.
In addition, Japanese Patent Laid-Open Publication Nos. 57812, 5-88385, etc. propose various methods for solving the sagging problem. These methods, however, each need a complicated apparatus and is apt to result in an irregular film.
As for a photoconductive element for electrophotography, use is often made of a coating liquid containing methylene chloride or similar chloric solvent. Chloric solvents, however, are harmful to the environment in various ways, e.g., ozone-unfriendly and should not be used wherever possible. Considering the solubility of binder resin and safety, there can be used only a small number of solvents including tetrahydrofurane as a solvent for a photoconductive coating liquid. However, such solvents have a problem that their boiling points are lower than the boiling points of chloric solvents customary with photoconductive elements. As a result, the resulting film needs a long period of time to dry and sags more than the conventional film at its upper portion.
Further, when the photoconductive element is applied to a printer or a facsimile apparatus by way of example, the photoconductive film formed thereon wears due to repeated operation and reduces the service life of the element. In light of this, it has been proposed to use a wear resistant, high-molecular substance as the resin component of the photoconductive film or to provide a charge transfer layer with wear resistance by polymerization. This kind of scheme, however, increases the viscosity of the coating liquid if the density of a solid component is the same as the conventional density. Should the density of a solid component be reduced, the liquid would noticeably sag in the upper portion of the film.
Japanese Patent Laid-Open Publication No. 9-265193 discloses a dip coating apparatus constructed to obviate the irregularity of the film on the base and the sagging of the liquid in the upper portion of the film. The dip coating apparatus includes a jig for holding the base and including an air outlet implemented by a cylindrical porous body. Air sent from the air outlet is caused to flow downward toward the surface of a coating liquid. Even this kind of apparatus renders a film formed on the base irregular or rough when applied to the production of a photoconductive element for electrophotography.